Cumene, or isopropylbenzene, is a valuable product that is used mainly for the manufacture of phenol and acetone. Cumene has been produced by a catalytic process using a solid phosphoric acid that is made by impregnating kieselguhr with phosphoric acid. Now, zeolitic catalysts are used to produce higher quality cumene at a lower investment cost.
In a typical commercial process for the production of cumene, liquid benzene and liquid propylene are charged into an alkylation zone containing one or more reactors containing an alkylation catalyst. In order to minimize the production of polyalkylated products of benzene it has been the practice to maintain a molar excess of benzene throughout the reaction zone ranging from about 4:1 to about 16:1, and more preferably about 8:1 of benzene to propylene. Two competing reactions with the desired production of isopropylbenzene have created problems in some commercial processes. One of these has been the formation of polyalkylated benzenes such as di- and triisopropylbenzene rather than the desired monoalkylated product. This competing reaction may be partially controlled by employing large molar excesses of benzene. However, a transalkylation reactor is employed to react polyalkylated benzenes with benzene to form additional cumene. The other competing reaction causing losses in the yield of cumene based on propylene reactant charged is the formation of oligomers of propylene such as propylene dimer and trimer which occur to a limited extent even with the large molar excesses of benzene present. Propylene trimers and some of the propylene tetramers boil with cumene. Since the presence of these olefins interfere with the oxidation reaction used to make phenol from cumene, this oligomerization side reaction must be minimized to make a high purity product.
The alkylator and transalkylator effluents undergo separation operations to separate benzene, cumene product, polyisopropylbenzene, and by-product streams using distillation columns. Traditionally three distillation columns are used. The first is typically a benzene column, used to recover excess benzene from the reactor effluents. The benzene column overhead, which is largely benzene, is typically recycled to the alkylator and transalkylator. The second distillation column is typically a cumene column used to recover the cumene product from the benzene column bottoms. The cumene product is typically the net overhead from the cumene column. The cumene product may be used in applications such as phenol or acetone processes, or may be sent to storage. The third distillation column is usually a polyisopropylbenzene column used to recover polyisopropylbenzene recycle stream from the cumene column bottoms. Polyisopropylbenzene is recovered as overhead from the polyisopropylbenzene column and is typically recycled to the transalkylator. The high boiling bottoms, the heavy ends, is usually cooled and sent to storage.
Current process flow schemes are improved by replacing the benzene column and the cumene column with a single dividing wall column. The resulting advantages include a significant savings in the energy requirement and the total number of stages, a higher cumene purity and a reduction in benzene loss. Additional advantages include a reduction in capital costs associated with the reduced number of stages, reduction in exchanger surface area, and a reduction in the equipment count.
The dividing wall or Petyluk configuration for fractionation columns was initially introduced some 50 years ago by Petyluk et al. A recent commercialization of a fractionation column employing this technique prompted more recent investigations as described in the article appearing at page s14 of a Supplement to The Chemical Engineer, 27 Aug. 1992.
The use of dividing wall columns in the separation of hydrocarbons is also described in the patent literature. For instance, U.S. Pat. No. 2,471,134 issued to R. O. Wright describes the use of a dividing wall column in the separation of light hydrocarbons ranging from methane to butane. U.S. Pat. No. 4,230,533 issued to V. A. Giroux describes a control system for a dividing wall column and illustrates the use of the claimed invention in the separation of aromatics comprising benzene, toluene and orthoxylene.
Using a dividing wall column in the a cumene production process provides significant advantages over cumene production processes that do not employ a dividing wall fractionation column, as shown below.